Symmetrize Along Custom Transform Orientation
It works fine for me. I guess it's the matter of the proper placing of the object's origin point.


You may also try mirroring with Empty. In Edit Mode select the face you want to add a custom transformation to, press Shift+Alt+Spacebar to set it. Then press Shift+Num 7 to allign the face to view. Enter Object Mode, add an Empty and check the Align to View checkbox. Add a Mirror Modifier and set the Empty as a mirror object's target. Now translate the Empty or the object along your custom transformation.